1. Field of the Invention
The present invention pertains generally to servo control systems and particularly to a servo control system for a reciprocating piston respirator.
2. Description of the Background
Within the past few years significant advancements have been made in reciprocating piston respirators. For example, U.S. Pat. No. 4,493,614 issued Jan. 15, 1985, to Chu et al, entitled "Pump for a Portable Ventilator" and assigned to Lifecare Services, Inc., Boulder, Color., discloses a piston cylinder arrangement in which a piston is reciprocally moved within the cylinder by a rotating shaft. A threaded shaft engages a threaded coupling device mounted in the piston so that linear movement is produced in the piston in response to the rotation of the threaded shaft. This linear relationship between the rotation of the motor in the movement of the piston comprises a basis for producing precise output flow characteristics from the reciprocating piston respirator. U.S. Pat. No. 4,493,514, cited above is specifically incorporated herein by reference for all that it discloses.
A patentability search was performed on this invention and the following U.S. Patents were uncovered.
______________________________________ Inventor U.S. Pat. No. Issue Date ______________________________________ McGuire 3,610,782 October 5, 1971 Perkins et al 4,276,003 June 30, 1981 Thomas et al 4,384,825 May 24, 1983 Schneider et al 4,498,843 February 12, 1985 ______________________________________
U.S. Pat. No. 4,384,825 issued May 24, 1983 to Thomas et al discloses a pump having a speed control circuit 16 which senses the motor speed of the pump drive motor 12 and provides an error signal to drive the motor 12 to maintain the pump speed constant.
U.S. Pat. No. 4,498,843 issued Feb. 12, 1985 to Schneider et al discloses a microprocessor 118 that senses the rotational speed of the pump output shaft using a optical encoding wheel and develops a driving voltage for electric motor 12.
U.S. Pat. No. 3,610,782 issued Oct. 5, 1971 to McGuire discloses a pump having a speed control servo which utilizes a variable pulse rate generator to vary the rate of the pump.
U.S. Pat. No. 4,276,003 issued June 30, 1981 to Perkins et al discloses a reciprocating piston pump wherein a reciprocating bearing mating with screw threads on the piston shaft cause the reciprocating piston to move. The motor control device 56 controls the movement of the piston.
Microprocessor control of the movement of a piston and a reciprocating piston pump has been specifically disclosed in the Schneider et al patent. Although such systems, such as disclosed in the Schneider et al patent are capable of controlling movement of a piston, such systems have been unable to precisely control the movement of the piston with a high degree of accuracy and simultaneously provide multiple output flow profiles. Conventional servo control methods employ classical control system analysis techniques which utilize linear feedback control methods to reduce error in the system. The classical techniques of control system analysis are disclosed in detail in "Elements of Control System Analysis; Classical and Modern Approaches" by Chih-Fan Chen and I. John Haas, Prentice Hall, Inc., New Jersey and "Automatic Control Systems" by Benjamin C. Kuo, 2nd Ed., Prentice Hall, Inc., New Jersey, both of which are specifically incorporated herein by reference for all that they disclose.
These classical techniques of control system analysis utilize frequency domain transformations of linear models which typically employ Laplace transformations. Although such classical linear analysis techiques are useful in servo control systems, such techniques are based upon reduction of an error signal from previous data. Consequently, such systems are based on "non-predictive" analysis techniques.
In order to implement conventional control system techniques, feedback systems are utilized which have inherent hysteresis to prevent undampened oscillations. The designed hysteresis of the system results in a built-in error factor at all times. The error factor greatly reduces the accuracy of the system.
Moreover, classical linear frequency domain control system analysis techniques rely upon detected velocity data as a basis for analysis and control. Instantaneous velocity signals in low velocity systems are extremely difficult to obtain. Low velocity tachometers for obtaining velocity information are extremely expensive and normally very inaccurate. The inaccuracy of such detection devices further adds to inaccuracies in the application of classical analysis techniques to low velocity servo systems.
Hence, classical techniques of servo control are unsuited for controlling the movement of the piston in a reciprocating piston respirator.